1. FIELD OF THE INVENTION
This invention relates to a disposable safety syringe having a cannula lock for releasably retaining a needle cannula in an axially extended position at which an injection may be administered, and, more particularly, to engageable piston and plug members which are movable reciprocally through the syringe cylinder to expulse a fluid medication therefrom and to retract the cannula from the cannula lock for relocation to the interior of said cylinder.
2. PRIOR ART
In U.S. patent application Ser. No. 211,366 filed June 24, 1988, which application has been or will be assigned to the assignee of the present application, a combination retractable hypodermic needle cannula and cannula lock is disclosed for a reusable safety syringe. The aforementioned combination cannula and lock is interfaced with a prefilled medication carpule at the interior of the reusable syringe cylinder. By virtue of the foregoing, an efficient locking means was disclosed by which a double ended needle cannula could be either reliably retained in an axially extended position for administering an injection or released from the axially extended position to be retracted within and surrounded by an empty medication carpule.
The invention disclosed in the present patent application is characterized by the same advantages as the invention disclosed in application Ser. No. 211,366. However, this application relates to a disposable safety syringe having a needle cannula which is retractable from an axially extended position for administering an injection to the interior of the syringe cylinder, rather than the interior of a medication carpule, so that the syringe may be discarded with the cannula shielded by and irretrievably located within the cylinder thereof. What is more, the present invention includes the additional advantage of a piston-plug assembly which is movable reciprocally through the syringe cylinder for expulsing the fluid contents thereof via said cannula and for engaging and reliably retaining one end of the cannula so that said cannula may be easily and efficiently retracted into the cylinder.